This application claims priority under 35 U.S.C. § 119 (a) on Patent Application No. 2005-62712 filed in Japan on Mar. 7, 2005, the entire contents of which are hereby implemented by reference.
a. Field of the Invention
The present invention relates to a high frequency module that is built into a high frequency device such as tuner, and the present invention more specifically relates to an improvement in an attachment structure of a circuit substrate and a shield case.
b. Description of the Related Art
In a high frequency module that is built into a high frequency device such as a tuner, an RF modulator, a switch box (SW-BOX), a voltage controlled oscillator (VCO) a temperature-compensated crystal oscillator (TCXO), a shield case is attached to a circuit substrate that has electronic components mounted thereon. Roles of this shield case include, for example, prevention of an influence of an outside electric field or magnetic field on a circuit that consists of electronic components and the like on a circuit substrate, as well as reinforcement of the mechanical strength of the circuit substrate, and prevention of an influence of a mechanical external force such as oscillation on a circuit that consists of electric components and the like on a circuit substrate. In addition, in a high frequency module in a radio device, a metal-made shield case is placed to cover the periphery of a circuit substrate in order to block electromagnetic waves from a high frequency circuit that is mounted on the circuit substrate.
As an example of a structure for attaching a shield case to such a circuit substrate, a structure as shown in FIGS. 6 and 7 has been used (for example, see JP H7-38267 (FIGS. 5 and 6)).
More specifically, a shield case 101 has a notch portion at an upper end portion 101a of a case side plate, and a leg portion 111 in the shape of a narrow horseshoe, with a tip end portion 111a wide and a base end portion 111b narrow, is formed within the notched portion. The shield case 101 is, then, set from the upper end portion (in FIG. 6A, set from below a circuit substrate 102 upwardly) so as to cover the periphery of the circuit substrate 102 and, as shown in FIG. 6B, the tip end portion 111a of the leg portion 111 is cocked (bent) inward using a processing jig or the like so that an end portion 102a of the circuit substrate 102 is held, and the tip end portion 111a of the leg portion 111 and a copper foil pattern (not shown) on the circuit substrate 102 are connected and fixed to each other with solder 103.
The reason why the circuit substrate 102 and the shield case 101 are connected and fixed in this manner is as follows. That is, after inserting an assembly part that has chip components mounted on the circuit substrate 102 into the shield case 101, the module needs to be flipped upside down to insert various insertion components, such as coils, a high frequency circuit (RFC), an intermediate frequency transformer (IFT) and a surface acoustic wave (SAW) filter, into the circuit substrate 102, and in order to prevent falling of the circuit substrate 102 at this time, the circuit substrate 102 and the shield case 101 need to be connected and fixed in advance.
Meanwhile, the circuit substrate 102 that has been connected and fixed to the shield case 101 as described above is, as shown in FIG. 7, provided with terminal board substrate portions 104 that extrude from the shield case 101 on one side thereof, and has a structure in which a terminal board 105 is attached to the bottom surface of the terminal board substrate portions 104. Reference numeral 106 in the drawing indicates an F connector.
As described above, in a conventional structure for attaching a shield case to a circuit substrate, a step for bending the leg portion 111 in the inward direction of the case is required when the shield case 101 is attached to the circuit substrate 102. For this reason, there is a problem in that the process man hour increases, which makes it difficult to simplify steps for producing a high frequency module. In addition, there is a problem in that a sufficient mounting strength of the solder 103 cannot be obtained due to a poor supply of the solder 103. Furthermore, when this high frequency module is installed, for example, on a TV, VTR, or DVD set device or the like, if the temperature of the environment for use greatly varies, for example from −20 degrees to +70 degrees (especially in winter, a large temperature difference is created between before and after turning on a device power), there is a problem in that a solder crack is caused by a stress between the leg portion 111 and the circuit substrate 102 since they have different thermal contraction rates.
In addition, in the markets for DVD, small-sized liquid crystal TV, and the like, on one hand, a short-type tuner is required, but on the other hand, circuit functions and electrical performance that are better than the conventional ones are usually required. For this reason, there has been a request for securing a larger mounting area for circuit components. However, with a conventional structure in which a terminal board for taking out terminals from a circuit substrate is attached to the bottom surface of the circuit substrate, there is a problem in that this portion of the circuit substrate cannot be used for mounting components, which practically decreases the mounting area of the circuit.